plothelper: New Plots Based on 'ggplot2' and Functions to Create Regular
Shapes

An extension to 'ggplot2' with miscellaneous functions. It contains
two groups of functions: Functions in the first group draw 'ggplot2' - based plots: gg_shading_bar() draws barplot
with shading colors in each bar. geom_rect_cm(), geom_circle_cm() and geom_ellipse_cm() draw rectangles, circles
and ellipses with centimeter as their unit. Thus their sizes do not change when the coordinate system or the aspect ratio
changes. annotation_transparent_text() draws labels with transparent texts. annotation_shading_polygon() draws irregular
polygons with shading colors. Functions in the second group generate coordinates for regular shapes and make linear transformations.